Analysis of Driving Factors on the Nitrogen Decrease in the Early Stage of the Thermal Stratification in Main Area of Zhoucun Reservoir

: In order to explore the trend and driving factors of nitrogen in the early stage of the thermal stratification in Zhoucun reservoir, the water quality indicators in main reservoir area of Zhoucun reservoir were monitored regularly form February to April (2016). Meanwhile, the fresh water and surface sediments in the sampling point were collected to study the effect of water and sediment denitrification by simulation in laboratory. The results showed that: the concentration of TN decreased from (2.28±0.09) mg·L-1 to (1.08±0.09) mg·L-1, the concentration of nitrate nitrogen decreased from (1.66±0.09) mg·L-1 to (0.25±0.06) mg·L-1, whereas the concentration of ammonia nitrogen and nitrite showed little change in the early thermal stratification of Zhoucun reservoir. At the same time, chlorophyll was not significantly increased, so the algae had little effect on nitrogen; the decreasing nitrogen was mainly due to the aerobic denitrification. Gradually increased temperature,the changes of DO and pH and the composition of organic matter (low molecular weight) were beneficial to the growth of aerobic denitrifying bacteria. Furthermore, the number of bacteria increased from 1.06×105 cfu·L-1 to 8.33×106 cfu·L-1, which enhanced the denitrification of the reservoir; Meanwhile, in the simulation experiments, the TN removal rates of water and water-sediment systems reached 0.7 mg and 3.3 mg, respectively. Water and sediment denitrification ratio was generally 1:4, and the aerobic denitrification of surface sediment was an important factor in the removal of nitrogen in reservoir.